Butanol is an important industrial chemical, useful as a fuel additive, as a feedstock chemical in the plastics industry, and as a foodgrade extractant in the food and flavor industry. Each year 10 to 12 billion pounds of butanol are produced by petrochemical means and the need for this commodity chemical will likely increase.
Butanol may be made through chemical synthesis or by fermentation. Isobutanol is a component of “fusel oil”, which can form under certain conditions as a result of incomplete metabolism of amino acids by yeast. Under some circumstances, isobutanol, may be produced from catabolism of L-valine. (See, e.g., Dickinson et al., J. Biol. Chem. 273 (40): 25752-25756 (1998)). Additionally, recombinant microbial production hosts, expressing an isobutanol biosynthetic pathway have been described. (Donaldson et al., commonly owned U.S. Pat. Nos. 7,851,188 and 7,993,889).
Efficient biological production of butanols may be limited by butanol toxicity to the host microorganism used in fermentation for butanol production. Accordingly, there is a need for genetic modifications which may confer tolerance to butanol.